It is common to employ an electromagnetic feed horn to illuminate a reflector antenna. Desirable performance characteristics are similar for all conical horns having either circular or square apertures used with either parabolic or spherical reflectors.
Such horns typically form the termination of a waveguide transmission system and are thus the final impedance matching component between the waveguide and the reflector. In providing the transition from reflector to waveguide propagation, the feed horn should not overly attenuate, nor introduce excessive noise onto the signal being received or emitted. Signal gain of the horn is determined by the size of its aperture characteristics, while the amount of noise introduced by the horn is partly related to the asymmetry of the radiation patterns in the E- and H-planes.
While the signal illuminating the reflector antenna is typically tapered to provide a good compromise between signal-to-noise ratio and gain, unmodified feed horns have unequal E- and H-plane beamwidths. The unequal beamwidths arise because the E-plane radiation, which tends to "fringe" or bend around the edges of the horn aperture, is larger than the aperture dimension, and the H-plane radiation, which is sinusoidally distributed across the aperture of the horn, is the same as the physical dimension of the horn because no current flows in the walls of the horn.
In the past, E- and H-plane radiation symmetry has been achieved for the special case of vertically polarized excitation by making the aperture of a conical horn rectangular, where the vertical direction is the direction parallel to a short edge of the rectangle, and the H-plane beamwidth is the length of the long edge of the rectangle and substantially equal to the E-plane pattern. However, increasing the H-plane dimension to widen the pattern is not a satisfactory solution because the E-plane radiation continues to fringe and energy is lost along the outside of the horn and because E- and H-phase centers are shifted with respect to each other. Thus well-focused, circularly polarized response from such horns is impossible.
Both rectangular and circular aperture feed horns used with short f/D reflectors, where f is the focal length of the reflector and D is the aperture diameter of the reflector and their ratio is greater than or equal to one, may be easily modified to equalize E- and H-plane radiation by using radially scalar or corrugated structure around the periphery of the aperture. Such corrugation substantially prevents E-field fringing and resultant losses and asymmetry of E- and H-plane beamwidths. However, for the case of long f/D antennae, radially corrugated structures become too large to be practical.
Long f/D antennae are more desirable and becoming more widely used in applications where less discrete focal points are desirable. Such antennae are typically used where the signal sources may be moved or where more than one signal source is to be received by the same antenna. In such applications, it is undesirable to move the entire antenna to receive the signals; rather, it is preferable to move only the feed horn, or, in the case of multiple signal sources, to provide more than one feed horn at a non-discrete focal point.
A feed horn constructed according to the principles of the present invention equalizes E- and H-plane radiation when used with long f/D reflectors and comprises four elements. One element is an aperture, either circular or rectangular, scaled to provide tapered illumination of the reflecting surface. Another element is one-quarter wavelength protuberances symmetrically disposed along the entire periphery of a circular aperture, and along the edges of the rectangular aperture which terminate the E-plane radiation. The diameter and spacing of the protuberances are determined empirically for each aperture. Such protuberances effectively prevent E-plane radiation from fringing and, in practice, substantially equalize the E- and H-plane beamwidths. In addition, the feed horn of the present invention provides a circular waveguide transition from the taper of the conical horn to convert energy flowing down the horn into waveguide propagation mode. Where circular polarization is desired, the waveguide transition section may include field retarding posts. Finally, a feed horn according to the present invention includes an impedance matching section in which circular waveguide energy is provided with suitable impedance matching to facilitate waveguide propogation at the selected frequency band.